Electrical penetration assemblies are utilized to pass electrical conductors through containment vessels of nuclear power generating stations. The wall of the containment vessel, which is a relatively thick concrete combined with a steel dome, has a steel penetration nozzle which penetrates the wall. The nozzle, on its outer end, has a flange suitably welded thereto and may have a similar flange on the interior end. The electrical penetration assemblies are basically of two types, the canister type and the unitized header type. In the canister type, two header plates, each containing several feed-through modules, are mounted and sealed on the opposing flanges of a nozzle to form a canister wherein the inside may be pressurized with dry nitrogen to monitor any gas leakage out of the nozzle. In the unitized header electrical penetration assembly, a header plate is provided with one or more conductors or modules (i.e., a conductor surrounded with insulation which is sealed thereto). The modules are suitably affixed to the header plate which is bolted or fixed onto the exterior flange of the nozzle. Porting holes or narrow passageways interconnect all of the modules in the header plate thereby providing means to monitor any gas leakage.
Typical unitized header electrical penetration assemblies are taught by U.S. Pat. Nos. 3,781,453 and 3,828,118. The system described in 3,781,453 consists of a canister containing two epoxy resin bushings having the conductors fixed at each end and the conductors are spliced internally of the canister. The system described in U.S. Pat. No. 3,828,118 consists of modules made of glass-reinforced, anhydride-cured epichlorohydrin bisphenol A-type epoxy resin with conductors bonded therebetween. Anhydride-cured epoxy resin undergoes reversion due to hydrolysis when subjected to super heated steam as would be encountered during an accident to a reactor system. Furthermore, such resins show considerable degradation under thermal aging simulating 40 years of design life and show a decrease in strength and flexibility when subjected to gamma radiations of over 100 million rads.
A typical canister type is taught by U.S. Pat. No. 3,856,983 wherein each conductor passes through a seal means on each end of the penetration nozzle. As is obvious by studying this patent, canister types are inherently complicated and costly.
Although the prior art teaches the bolting of a header plate onto the flange of a penetration nozzle for unitized header types, the prior art does not teach how the seal therebetween is to be monitored for leaks.